At present, in the printing sector, and in particular in the sector relating to the printing of labels and flexible wrappers, flexographic printing machines which have been adapted with ink-jet printing heads are known. The application of ink-jet printing heads to flexographic printing units allows printing to be personalized with so-called “variable data”, said data ensuring that repetition of a print is not the same as the next print, modifying in succession one print in relation to the other one.
Also known are indirect ink-jet printing machines which perform a first transfer of the image onto an intermediate medium, using the ink-jet printing method. The intermediate medium moves at the same line speed as the printing medium. When the intermediate medium comes into contact with the printing medium, the intermediate medium, performs ink transfer by the offset printing method. At the time of transfer from the intermediate medium to the printing medium, the ink is still liquid or in semi-liquid.
The advantages of indirect printing together over those of ink-jet printing are many. For example, it is possible to perform printing on a greater variety of printing media types owing to the possible application of any pressure to the ink transfer step, as in flexographic printing and, in particular, offset printing.
However, this type of machine also has certain drawbacks. In particular, it is affected by the problem of latent image, namely the defect which arises on the printing medium when complete transfer of the ink during the previous step is not obtained. A way of overcoming this problem is to perform calibration between the ink-jet repetitions, namely ensure that each ink drop of a successive repetition is transferred exactly onto the same point of the preceding repetition, within the same image portion. However, this would involve having to adapt the mechanical system of the image transfer system, namely the intermediate medium, to the specific format of the printed images. This type of operation among other things would require very long set-up times which would cancel out the advantages of ink-jet printing.
In order to avoid this, namely ensure that the mechanics of the image transfer system are independent of the dimensions of the print format, as in the case of flexographic printing machines, cleaning systems have been implemented in order to clean the surface of the intermediate printing medium and eliminate the latent image defect without having to re-calibrate the mechanics and in particular the arrangement of the intermediate medium.
However, these cleaning systems also have certain drawbacks. They are in fact costly and often inefficient. Furthermore they require a lot of maintenance.